The present invention relates to a method of attaching macro-objects to an electrically conducting or semiconducting surface by means of electrografting, and also to the electrically conducting or semiconducting surfaces obtained using this method and to applications thereof.
The production of electrically conducting or semiconducting surfaces covered with macro-objects and in particular with polymer films is of great interest in many fields, in particular for the manufacture of electronic components or of integrated optical devices, for producing devices that can be used in the biomedical field or in biotechnology (DNA chips, protein chips, etc.), for anticorrosive protection, and also for any modifications of the superficial properties of metals or of semiconductors.
It appears to be accepted, today, that the obtaining of grafted polymer films by electrografting of activated vinyl monomers on conducting surfaces takes place by means of electroinitiation of the polymerization reaction from the surface, followed by chain growth, monomer by monomer. The reaction mechanism of electrografting has in particular been described in the articles by C. Bureau et al., Macromolecules, 1997, 30, 333; C. Bureau and J. Delhalle, Journal of Surface Analysis, 1999, 6(2), 159 and C. Bureau et al., Journal of Adhesion, 1996, 58, 101.
By way of example, the reaction mechanism for the electrografting of acrylonitrile by cathode polarization can be represented according to SCHEME 1 below, in which the grafting reaction corresponds to step No. 1, where the growth takes place from the surface; step No. 2 being the main parasite reaction which results in a nongrafted polymer being obtained:
1: Surface Chemical Reaction, Grafting
2: Desorption, Polymerization in Solution
The growth of the grafted chains therefore takes place by means of purely chemical polymerization, i.e. independently of the polarization of the conducting surface which has given rise to the grafting. This step is therefore sensitive to (it is in particular interrupted by) the presence of chemical inhibitors of this growth, in particular through protons.
In scheme 1 above, in which the electrografting of acrylonitrile with cathode polarization is considered, the grafted chain growth takes place by anionic polymerization. This growth is interrupted in particular by protons, and it has been demonstrated that the proton content even constitutes the major parameter which controls the formation of polymer in solution, and also the information recovered in the course of synthesis, in particular the appearance of the voltammograms which accompany the synthesis (see in particular the article by C. Bureau, Journal of Electroanalytical Chemistry, 1999, 479, 43).
Traces of water, and more generally labile protons from protic solvents, constitute sources of protons that are prejudicial to the growth of the grafted chains.
Even before the reaction mechanisms for the electrografting of vinyl monomers were understood, this technical stumbling block had been clearly identified by those skilled in the art, as witnessed by the details of the various methods developed on the basis of these compounds:                in patent application FR-A-2 480 314, the authors mention a method of electrografting vinyl monomers consisting in preparing a solution having a water content at most equal to 10−3 mol/l, and even specify, in a preferred embodiment, that this water content should at most be equal to 5×10−4 mol/l;        in patent application EP-A-0 618 276, the authors mention a method of electrografting vinyl monomers using an aprotic solvent;        in patent application EP-A-0 665 275, the authors also mention a method of electrografting vinyl monomers using aprotic organic solvents. In addition, the description section of that prior application specifies that the water content of the electrolysis bath is preferably less than 10−3 M. Thus, and before electrolysis, the electrolysis bath is degassed by bubbling through an inert gas containing at most 5 ppm of water and 10 ppm of oxygen;        in U.S. Pat. No. 6,180,346, the authors use a method of electropolymerization of molecules comprising vinyl substituents. By way of example, they mention the use of acetonitrile as solvent and specify that this should be dried before use, which, for those skilled in the art, reflects a residual water content of the order of a few tens of ppm at most.        
The very low water contents required by the electrografting of vinyl monomers are in general maintained, according to the teaching of the documents of the prior art mentioned above, during or before synthesis, via various dehydration techniques such as, for example, via sparging of dry inert gas (nitrogen, argon, etc.), the water content of which is of the order of a few ppm.
For similar reasons of control of the content of proton sources in the reaction medium, only aprotic solvents, and monomers which are themselves aprotic, i.e. which do not comprise functional groups having acid functions (in the Brönsted sense) in the study solvent, have been proposed for obtaining electrografted organic films.
In practice, the water content of these solvents is decreased at the cost of a long and laborious preparation, for example via a period of time on dehydrating compounds such as phosphorus pentoxide (P2O5) or on molecular sieves (for example with a porosity of 4 ångströms), via distillation under reduced pressure of rare inert gases (nitrogen, argon, etc.) or via a combination of these methods. It is thus noted:                in patent application FR-A-2 480 314, that the authors recommend the use of an aprotic organic solvent which does not give a parasite reaction with the monomer used;        in patent application EP-A-0 618 276, the authors also recommend the use of aprotic solvents; and        in patent application EP-A-0 665 275, besides mentioning the use of aprotic solvents, the authors state the monomer structures that can be used and specify that the possible protic functional groups of the monomer(s) should be pre-masked.        
In practice, the monomers used for electrosynthesis are distilled before use, so as to remove various additives, and in particular the polymerization inhibitors added by the manufacturer in order to stabilize the product and to avoid its polymerization in the bottle under storage conditions.
It will be noted only that patent application EP-A-0 665 275 mentions the use of specific inhibitors so as to be able to introduce new functionalities at the end of the growing polymer chains. However, it has been demonstrated, in particular in the article by C. Bureau et al., 1996 (previously mentioned), that the growth of the polymer chains on the surface is necessarily anionic, and it is probable that the radical inhibitors introduced by the authors are in the film at the end of synthesis because they are adsorbed and/or reduced on the surface of the electrode (they are in general electroactive), and not because they interrupt the chain growth as indicated in patent application EP-A-0 665 275.
Finally, and most recently, there has also been proposed a method of forming polymer films on an electricity-conducting surface by electrografting, in anhydrous medium, of preformed polymers, and in particular of polyesters, these polymers being functionalized with electrograftable pendant groups such as acrylate groups (Lou et al., American Chemical Society, Langmuir, ASAP articles, 2002, available on the Internet at the following address: http://pubs.acs.org/isubscribe/journals/langd5/pdf/la011289g.pdf; C. Detrembleur et al., Angewandte Chemie International Edition in English, 2001, 40, 1268). According to the authors, this electrografting is carried out under strictly anhydrous operating conditions. In addition, in order to obtain a film of good quality, the authors indicate that the polymer intended to be electrografted should comprise a minimum number of pendant electrograftable groups for the adsorption of the polymers to the metal surface to be able to take place. According to that article, this minimum number appears to correspond to a degree of substitution of greater than approximately 40%, i.e. 40% of the monomer units of the polymeric chain carry an acrylate group. Finally, it is also indicated that, if the concentration of acrylate of the preformed polymer in the electrolyte solution is greater than a certain value, then no electrografting takes place. This limiting concentration appears to depend on the degree of substitution of acrylate groups along the chain, but appears to be of the order of 3 mol/l of acrylate groups.
Due to these strict limitations relating at once to the solvents and the types of monomers of synthesis, to the water content in the reaction medium and, finally, to the number and the concentration of the electrograftable groups to be used, the electrografting of vinyl monomers or of preformed polymers comprising pendant electrograftable groups, to electrically conducting or semiconducting surfaces, remains a process that it is difficult to apply on an industrial scale.
In addition, all these conditions—which therefore relate both to the nature of the monomer and to the reaction medium—are very restrictive since they exclude a considerable number of macromolecules from the list of compounds which can be attached by electrografting, and in particular:                macromolecules which are not polymers, in the strict sense of the term, i.e. which do not constitute a macromolecule resulting from the repetition of the same unit. Mention may be made, for example, of macromolecules of biological interest, such as oligonucleotides and single- or double-stranded DNA, single- or double-stranded RNA, oligopeptides, proteins and in particular enzymes, antibodies, or else growth factors, cellulose and modified celluloses, dextrans, chitosans and more generally polysaccharides, and also any macromolecules resulting from mixtures or from these base units (such as, for example, peptidoglycans, etc.). Mention may also be made of all the macro-structures that are nonpolymeric and not strictly macromolecular, such as, for example, those obtained by crosslinking of a two- or three-dimensional network, for instance rubber, etc., liposomes and all the macro-ions or the mineral aggregates of any types, such as, for example, silica beads, nano-objects, nanobeads, nanotubes, fullerenes, etc.), living cells, etc.;        polymers not resulting from the polymerization or from the copolymerization of vinyl monomers and/or of cyclic molecules. This concerns in particular all the polymers or copolymers obtained by condensation reactions, in particular when they carry protic groups, polymers or copolymers of lactic acid, of glycolic acid and of ε-caprolactone, nylon and more generally polyamides, polysiloxanes, poly(ortho esters), polymers or copolymers which can only be obtained by radical polymerization, whatever the nature of the monomer, such as polyisoprene or parylenes, or else polymers or copolymers obtained by a polymerization of Ziegler-Natta type, by Diels-Alder reactions, etc.        
Therefore, in order to remedy these major problems, the applicant has developed that which is the subject of the invention.
It in particular gave itself the aim of providing a method for electrografting all types of macro-objects onto electrically conducting or semiconducting surfaces, at water contents at least one hundred to one thousand times higher, with a simplified control of the atmosphere during synthesis, in solvents that are not necessarily aprotic, and with reagents and/or solvents that are not distilled, i.e. still containing their possible initiators.